hmacsha1 – Issue with implementing TOTP in PHP using hash_hmac()-ThrowExceptions

Exception or error:

I am trying to implement TOTP in PHP, and I get a six digit code, but it never matches the one in my Authenticator app. I was able to zero in on the problem being most likely related to the output of hash_hmac, since I tested all the bit shifting occurring after the hash is generated on the example in the RFC document and it produced the expected output. My question now is: What am I doing wrong? Am I passing the wrongly formatted input to hash_hmac? I am really at a loss here.

<?php

class TOTP
{
    function __construct ($d, $k)
    {
        $this->digits = $d;
        $this->secret = $k;
        $this->ival   = 30;
    }


    private function int2bytes ($n)
    {
        $bytes = array();

        for ($i = 7; $i >= 0; $i--) {
            $bytes[] = ($n >> ($i * 8)) & 0xff;
        }

        return pack("C*", ...$bytes);
    }


    function hotp ($d, $k, $c)
    {
        // @$d the number of digits
        // @$k the shared secret
        // @$c an integer counter

        $hmac  = hash_hmac("sha1", $this->int2bytes($c), $k);
        $hex   = str_split($hmac, 2);
        $bytes = array();

        for ($i = 0; $i < count($hex); $i++) {
            $bytes[] = hexdec($hex[$i]);
        }

        $offset  = $bytes[19] & 0xf;
        $bincode =
            ($bytes[$offset] & 0x7f) << 24 |
            ($bytes[$offset + 1] & 0xff) << 16|
            ($bytes[$offset + 2] & 0xff) << 8 |
            $bytes[$offset + 3] & 0xff;

        $hotp = strval($bincode % pow(10, $d));

        while (strlen($hotp) < $d) {
            $hotp = "0$hotp";
        }


        return $hotp;
    }


    public function get ()
    {
        $counter = floor(time() / $this->ival);

        return $this->hotp($this->digits, $this->secret, $counter);
    }
}

?>

Some example input and output for TOTP::get(), where $this->secret is always ” secret” and digits is always 6:

unix time: 1584041829
output:    79 06 09
expected:  91 97 63

unix time: 1584041867
output:    76 89 74
expected:  70 54 64

unix time: 1584041918
output:    46 57 96
expected:  52 96 15
How to solve:

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